1. Field of the Invention
The present invention relates to a valve timing control device that controls the valve timing through adjustment of the relative rotation phase of a crankshaft and a camshaft that is linked thereto in a driving manner.
2. Description of the Background Art
The approach disclosed in Patent Document 1 (Japanese Patent Laying-Open No. 2004-270488), for example, corresponds to such a conventional valve timing control device.
This device includes an electric motor qualified as the driving source of a variable mechanism that renders variable the relative rotation phase of a crankshaft and camshaft. When the valve timing is to be advanced, the rotation phase of the camshaft is modified towards the positive direction (forward direction) with respect to the crankshaft by driving the electric motor such that the rotational speed of the camshaft is increased. In contrast, when the valve timing is to be retarded, the rotation phase of the camshaft is modified towards the opposite direction with respect to the crankshaft by driving the electric motor such that the rotational speed of the camshaft is reduced.
There is also known a valve timing control device that controls the valve timing to attain an appropriate timing for startup when the internal combustion engine is to be started (for example, refer to Patent Document 2 (Japanese Patent Laying-Open No. 11-159311).
When valve timing control is to be executed in preparation to the startup of the engine, the valve timing at a device that allows valve timing control through an electric motor as disclosed in Patent Document 1 set forth above can be advanced or retarded by increasing or reducing the rotational speed of the electric motor according to a valve timing modification request.
The cam torque that increases and decreases periodically together with the rotation of the camshaft acts on the driving source of the camshaft. This cam torque is generated due to the elasticity of a valve spring acting on the cam. While the cam torque acts as a rotation resistance, the camshaft must be driven against this cam torque functioning as a resistance (resistance cam torque).
In the event of effecting valve timing control in preparation to engine startup in the mode of the electric motor employed as the driving source of a variable mechanism as set forth above, the variable mechanism will be motor-driven with the operation of the internal combustion engine stopped. This means that, when the valve timing is to be modified towards the side where the cam torque acts as a resistance with respect to the driving source, the electric motor must output torque of a level that can drive the variable mechanism against the resistance cam torque.
For example, assume that a valve timing modification request is made to rotate the camshaft until the resistance cam torque reaches the peak. In this case, valve timing modification corresponding to the aforementioned request can be effected by employing an electric motor that can output torque of a level that can drive the variable mechanism against the resistance cam torque attaining peak level. By employing such an electric motor, the valve timing can be modified whatever level the resistance cam torque may take within the variable range.
However, the requirement of a high output from the electric motor will impede downsizing of the electric motor, which in turn will impede downsizing of the internal combustion engine. The pursuit of such a high output that allows the variable mechanism to be driven against the resistance cam torque attaining the peak level raises concerns about the aforementioned problem becoming more noticeable.